There are many known procedures for producing small particles. Such particles are used in numerous applications including those using magnetic pigments and metallic catalysts.
One of the major processes in making small particles is that utilizing organometallic salts such as metal formates, oxalates and the like. These salts are usually thermally reduced to form metals. Occasionally, they may be further processed to produce the oxide of the metal moiety, e.g. to produce iron oxide, alumina or the like.
An improvement in this kind of process is described by Ehrreich and Reti in U.S. Pat. applications Ser. Nos. 228,387 now U.S. Pat. No. 3,892,673 and 367,461 now U.S. Pat. No. 3,843,349 filed Feb. 22, 1972, and June 6, 1973, respectively. The most important aspect of this work is the achievement of improved magnetic properties based on the use of a coating of an organic film-forming material on the particulate organometallic salt before it was subjected to reduction. A number of other workers including Haines, in U.S. Pat. No. 3,574,685 and Neel, in U.S. Pat. No. 2,651,105, have described work in this general field of organometallic salt reduction.
Other important work in this field is described in commonly-owned and co-pending U.S. Pat. application Ser. No. 509,467, filed Sept. 26, 1974 filed by Deffeyes on even date herewith and entitled HIGH POROSITY MATERIALS. In that application, it is shown that the Ehrreich-Reti technology can be improved to provide an important class of highly porous materials.
Also noted herein is a major advance, one made subsequently to the instant invention, disclosed in U.S. Pat. No. 3,909,240 filed on Sept. 28, 1973 by Deffeyes and Tyler et al. This is the use of complexing agents in the formulation of oxalates. Some advantageous structural characteristics imparted by use of the complexing agents to the organometallic salt survive the catastrophic decomposition of the salt and promote formation of metallic particles of particularly favorable magnetic chracteristics.
This latter process is mentioned here only because it, like the Ehrreich-Reti art, and the aforesaid Deffeyes art, is often used in the more advantageous embodiments of the instant invention to optimize the particulate material produced thereby.
In any event, at the time the instant invention was made, it remained a major problem in the art of making metallic powders to achieve a process whereby very small metallic particles could be manufactured which particles exhibit an improved combination of chemical stability and excellent magnetic properties or excellent catalytic properties.
After the invention to be described below was made, a hindsight review of the art relating to seeding was carried out. It was known to promote the crystallization in solution by adding seed crystals. The uniformity of refined sugar particles has long been promoted by use of sugar seed crystals. Moreover, sulfateprocess TiO.sub.2 particles have been influenced in size and shape by the introduction of seed crystals into the reaction medium. These are examples of processes wherein the seed crystal is introduced primarily for the purpose of providing a certain population density of crystal growth sites for the purpose of imparting readily identifiable characteristics to the crystals being manufactured.
A similar seeding process is described in U.S. Pat. No. 3,832,157 to KJELL et al whereby metal powders are formed directly by formation on seed crystals in a gaseous medium. The "seed crystals" in this process can be selected from such diverse materials as hydrogen gas, metal powders, or metal carbides. This process, too, is for the purpose of achieving the desirable number of crystal-growth sites and, thereby, a properly sized powder.
In the invention described below, it is to be emphasized that the seeding technique is utilized primarily to change the character, not of the crystal grown on the seed, but of decomposition products of the crystal. Moreover, it is not merely the observable characteristics of these metal-bearing decomposition products that are favorably affected: it is such esoteric properties as the catalytic and magnetic capabilities of the decomposition products that are enhanced.